1. Field of the Invention
The present application relates to a high electron mobility transistor (HEMT), in particular, relates to a HEMT made of primarily nitride semiconductor materials.
2. Background Arts
A Japanese Patent Application laid open No. JP2006-261642A has disclosed a field effect transistor (FET) and a process to form the FET. FIG. 4 schematically illustrates a cross section of the FET disclosed therein. The FET 100 provides an undoped GaN channel layer 102, an n-type AlGaN barrier layer 103, and an n-type InAlGaN contact layer 104 where these layers, 102 to 104, are sequentially grown on a substrate 101 made of sapphire (Al2O3). Provided on the n-type contact layer 104 are ohmic electrodes made of stacked metal of titanium (Ti) and aluminum (Al) for source and drain electrodes. A portion of the n-type contact layer 104 exposes the n-type barrier layer 103 on which another electrode 106, Schottky electrode, made of alloy containing palladium (Pd) and silicon (Si) is formed.
Another Japanese Patent Application laid open No. JP2015-037105A has disclosed a semiconductor device having InAlN layer. FIG. 5 schematically illustrates a cross section of a HEMT disclosed therein. The HEMT 200 includes GaN electron transporting layer 221 as the channel layer on a substrate 210, an InAlN electron supplying layer 222 as the barrier layer formed on the electron transporting layer 221, an AlGaN upper layer 223 formed on the electron supplying layer 222, a gate electrode 241 formed on the electron supplying layer 222 exposed within an opening of the AlGaN upper layer 223, and a source and a drain electrodes, 242 and 243, formed on the upper layer 223. The upper layer 223 and the electron supplying layer 222 in respective portions beneath the source and drain electrodes, 242 and 243, provide a region of the first conduction type implanted with impurities, for instance, silicon (Si).
Nitride semiconductor devices, because of wider bandgap energy thereof and accordingly greater breakdown voltages without decreasing a saturation electron velocity, have been widely investigated for applications of high power at high frequencies. In particular, a hetero-interface between GaN layer and AlGaN layer, or between GaN layer and InAlN layer, may induce enough electrons in the GaN layer in vicinity of the interface, which is often called as the two-dimensional electron gas (2DEG), and may be applicable to a high electron mobility transistor (HEMT).
High frequency performance of a transistor is determined by a cut-off frequency thereof. The cut-off frequency is the frequency at which the voltage gain of the transistor becomes the unity. In order to increase the cut-off frequency, the reduction of gate capacitance and the increase of the trans-conductance become key factors. The increase of the trans-conductance may be achieved by the reduction of the access resistance between the source electrode and the gate electrode. A thinner barrier layer and smaller contact resistance of the source electrode may be effective for the reduction of the access resistance.
An indium aluminum nitride (InAlN) is a most likely material for the barrier layer because even a thinner InAlN layer may induce greater electrons. Thus, the barrier layer made of InAlN may reduce the access resistance.
On the other hand, the reduction of the contact resistance has been left hard. In a HEMT primarily made of gallium arsenide (GaAs), a heavily doped contact layer, typically n+-GaAs layer, on the barrier layer made of AlGaAs, may effectively reduce the contact resistance. However, a HEMT made of nitride semiconductor materials, in particular, when a heavily doped GaN layer is provided on the InAlN barrier layer, and the source electrode is in contact to this heavily doped GaN layer; the interface between the InAlN barrier layer and the n+-GaN layer may also induce electrons, which raises the conduction band of the InAlN barrier layer and the InAlN barrier layer may operate as a barrier for the electron transport, which increases the access resistance.
An n+-type InAlGaN layer may be replaced from the n+-GaN layer as the first prior document. However, because InAlGaN contains indium (In), the n+-InAlGaN is necessary to be grown at relatively low temperature, which accelerates the capture of carbons (C) during the growth. Because carbons operate as an acceptor in nitride semiconductor materials, the InAlGaN layer is hard to be heavily doped with n-type impurities.
An ion implantation of n-type impurities may decrease the access resistance of a HEMT as the second prior document has disclosed. However, ions implanted deeper than the interface between the barrier layer and the channel layer may become sources for leak currents between the drain and source electrodes, which degrades the pinch-off performance of the HEMT.